Airway dysfunction in obesity: response to voluntary restoration of end expiratory lung volume.

INTRODUCTION: Abnormality in distal lung function may occur in obesity due to reduction in resting lung volume; however, airway inflammation, vascular congestion and/or concomitant intrinsic airway disease may also be present. The goal of this study is to 1) describe the phenotype of lung function in obese subjects utilizing spirometry, plethysmography and oscillometry; and 2) evaluate residual abnormality when the effect of mass loading is removed by voluntary elevation of end expiratory lung volume (EELV) to predicted FRC. METHODS: 100 non-smoking obese subjects without cardio-pulmonary disease and with normal airflow on spirometry underwent impulse oscillometry (IOS) at baseline and at the elevated EELV. RESULTS: FRC and ERV were reduced (44 ± 22, 62 ± 14% predicted) with normal RV/TLC (29 ± 9%). IOS demonstrated elevated resistance at 20 Hz (R20, 4.65 ± 1.07 cmH2O/L/s); however, specific conductance was normal (0.14 ± 0.04). Resistance at 5-20 Hz (R5-20, 1.86 ± 1.11 cmH2O/L/s) and reactance at 5 Hz (X5, -2.70 ± 1.44 cmH2O/L/s) were abnormal. During elevation of EELV, IOS abnormalities reversed to or towards normal. Residual abnormality in R5-20 was observed in some subjects despite elevation of EELV (1.16 ± 0.8 cmH2O/L/s). R5-20 responded to bronchodilator at baseline but not during elevation of EELV. CONCLUSIONS: This study describes the phenotype of lung dysfunction in obesity as reduction in FRC with airway narrowing, distal respiratory dysfunction and bronchodilator responsiveness. When R5-20 normalized during voluntary inflation, mass loading was considered the predominant mechanism. In contrast, when residual abnormality in R5-20 was demonstrable despite return of EELV to predicted FRC, mechanisms for airway dysfunction in addition to mass loading could be invoked.

Distal airway dysfunction in obese subjects corrects after bariatric surgery.

BACKGROUND: Obesity is frequently associated with respiratory symptoms despite normal large airway function as assessed by spirometry. However, reduced functional residual capacity and expiratory reserve volume are common and might reflect distal airway dysfunction. Impulse oscillometry (IOS) might identify distal airway abnormalities not detected using routine spirometry screening. Our objective was to test the hypothesis that excess body weight will result in distal airway dysfunction detected by IOS that reverses after bariatric surgery. The setting was a university hospital. METHODS: A total of 342 subjects underwent spirometry, plethysmography, and IOS before bariatric surgery. Of these patients, 75 repeated the testing after the loss of 20% of the total body weight. The data from 47 subjects with normal baseline spirometry and complete pre- and postoperative data were analyzed. RESULTS: IOS detected preoperative distal airway dysfunction despite normal spirometry findings by an abnormal airway resistance at an oscillation frequency of 20 Hz (4.75 ± 1.2 cm H2O/L/s), frequency dependence of resistance from 5 to 20 Hz (2.20 ± 1.6 cm H2O/L/s), and reactance at 5 Hz (-3.47 ± 2.1 cm H2O/L/s). Postoperatively, the subjects demonstrated 57% ± 15% excess weight loss. The body mass index decreased (from 44 ± 6 to 32 ± 5 kg/m2, P < .001). Improvements in functional residual capacity (from 59% ± 11% to 75% ± 20% predicted, P < .001) and expiratory reserve volume (from 41% ± 20% to 75% ± 20% predicted, P < .001) were demonstrated. Distal airway function also improved: airway resistance at an oscillation frequency of 20 Hz (3.91 ± .9, P < .001), frequency dependence of resistance from 5 to 20 Hz (1.17 ± .9, P < .001), and reactance at 5 Hz (-1.85 ± .9, P < .001). CONCLUSION: The present study detected significant distal airway dysfunction despite normal preoperative spirometry findings. The effect of increased body weight was likely the main mechanism for these abnormalities. However, the inflammatory state of obesity or associated respiratory disease could also be invoked. These abnormalities improved significantly toward normal after weight loss. The results of the present study highlight the importance of bariatric surgery as an effective intervention in reversing these respiratory abnormalities.

Disparity between proximal and distal airway reactivity during methacholine challenge.

There is an increasing awareness of the role of distal airways in the pathophysiology of obstructive lung diseases including asthma and chronic obstructive pulmonary disease. We hypothesize that during induced bronchoconstriction: 1) disparity between distal and proximal airway reactivity may occur; and 2) changes in distal airway function may explain symptom onset in subjects with minimal FEV(1) change. 185 subjects underwent methacholine challenge testing (MCT). In addition to spirometry, oscillometry was performed at baseline and after maximum dose of methacholine; 33/185 also underwent oscillometry after each dose. Oscillometric parameters included resistance at 5 and 20 Hz (R(5), R(20)) and heterogeneity of distal airway mechanics assessed by frequency dependence of resistance 5-20 Hz (R(5-20)) and reactance area (AX). R(5) varied widely during MCT (range -0.8 - 11.3 cmH(2)O/L/s) and correlated poorly with change in FEV(1) (r = 0.17). Changes in R(5) reflected changes in both R(20) and R(5-20) (r = 0.59, p<0.05; r = 0.87, p<0.0001). However, R(20) increased only 0.3 cmH(2)O/L/s, while R(5-20) increased 0.7 cmH(2)O/L/s for every 1cmH(2)O/L/s change in R(5,) indicating predominant effect of distal airway mechanics. 9/33 subjects developed symptoms despite minimal FEV(1) change (<5%), while R(5) increased 42% due to increased distal airway heterogeneity. These data indicate disparate behavior of proximal airway resistance (FEV(1) and R(20)) and distal airway heterogeneity (R(5-20) and AX). Distal airway reactivity may be associated with methacholine-induced symptoms despite absence of change in FEV(1). This study highlights the importance of disparity between proximal and distal airway behavior, which has implications in understanding pathophysiology of obstructive pulmonary diseases and their response to treatment.